A third of the seafood sold nationwide and almost 40 percent of the fish purchased by consumers in Northern California was not what it was touted to be, a study released Thursday by the nonprofit group Oceana revealed.

Genetic testing of 1,215 fish taken from 674 retail outlets, grocery stores and sushi bars throughout the United States between 2010 and 2012 found that 33 percent of the samples had been mislabeled, according to U.S. Food and Drug Administration guidelines.

Researchers with Oceana, a group dedicated to preserving the ocean ecosystem, reported finding seafood mislabeling in all 21 states where they tested, including retail outlets in San Francisco, Monterey, Los Angeles, Seattle and Portland.

In Northern California, 38 percent of the fish tested was something other than advertised. Fish labeled snapper turned out to be rockfish 34 times at sites in the Bay Area, Monterey and other sites in Northern California, according to the watchdog group, which did not reveal the locations of the specific venues where seafood fraud was found.

Restaurants in Northern California misidentified fish 58 percent of the time, while grocery stores mislabeled their products 27 percent of the time.

"Seafood fraud harms not only the consumer's pocket book, but also every honest vendor or fisherman along the supply chain," wrote the report's authors, Kimberly Warner, Walker Timme, Beth Lowell and Michael Hirshfield. "These fraudulent practices also carry potentially serious concerns for the health of consumers, and for the health of our oceans and vulnerable fish populations."

Legal mismatches

Misbranding food for financial gain is illegal under state and federal law, but in most cases there is no way for the consumer to know whether the fish is what the restaurant, fish market or grocery store claims it is. The Food and Drug Administration recognizes 1,700 marketable fish and publishes a guide of acceptable market names for certain species, but state and federal laws sometimes don't match up. California law allows 13 species of rockfish to be sold as Pacific red snapper, but FDA guidelines allow only one. The federal agency lists 14 species that can be labeled "tuna."

FDA regulators announced in 2011 that they would begin DNA testing at selected sites, but there is no nationwide system that monitors seafood fraud.

Obtaining samples

Volunteer fish sleuths armed with testing kits took the fish samples, and Oceana paid between $20 and $200 to conduct genetic sequencing on the tissues. The cost depended on the purity of the sample, which could be degraded by such things as lemon sauce, according to researchers.

The results show that Southern California has one of the highest mislabeling rates in the nation, with 52 percent of the fish testing out to be some other species. Pennsylvania was the worst, with 56 percent of the tested fish turning out to be something other than the label or menu stated, the report said.

Eight out of nine sushi samples labeled as "white tuna," or shiro maguro, at venues in the Los Angeles area were actually escolar. Fisheries biologists often call escolar the "ex-lax fish" for its often purgative effect on the digestive system. It is not among the 14 species that can legally be labeled as tuna.

Shark meat has also been found in fish tacos sold in the Golden State, a major concern, according to researchers, because, among other reasons, sharks often contain high concentrations of mercury. Serious illnesses have occurred in the past, most notably in 2007 when toxic puffer fish were mislabeled as monkfish in an attempt to circumvent U.S. import restrictions, according to the FDA.

Sushi biggest problem

There was mislabeling in 44 percent of the retail outlets nationwide. Sushi venues had the worst problem, with 74 percent of fish proving to be something other than what was advertised. Sushi places in Northern California served mislabeled fish 76 percent of the time. In New York, Washington D.C., Chicago and Texas, every sushi venue was selling mislabeled fish. In Houston and Austin, every single sample was mislabeled.

Restaurants sold mislabeled fish 38 percent of the time and grocery stores did it 18 percent of the time, according to the study.

Snapper was the most commonly misrepresented in the study, with 87 percent of it turning out to be something else, most often tilapia. Only seven of the 120 samples of fish touted as "red snapper" turned out to be that species. Tuna was switched out 59 percent of the time, making it the second most mislabeled fish.

Halibut, grouper, cod and Chilean sea bass were also commonly mislabeled. Salmon was switched out 7 percent of the time, according to the testing. That's at least partly because 2012 was the best wild salmon fishing season in years off the California coast, the authors said.

Escolar and tilapia weren't the only species that were being passed off as a more expensive fish. Overfished Atlantic cod, imperiled Gulf grouper and King mackerel, which is on the FDA's "do not eat" list because of high levels of mercury, were sold under false labels, the report stated.

The report said more government oversight and enforcement of the global seafood supply chain is needed if seafood fraud is ever going to be brought under control.

"It is difficult to determine if fraud is occurring at the boat, during processing, at the wholesale level, at the retail counter or somewhere else along the way," the authors concluded. "Our findings demonstrate that a comprehensive and transparent traceability system - one that tracks fish from boat to plate - must be established at the national level."